Prius 1.8L Brief Rattling at Startup: Head Gasket or EGR Issue?

[Jollyno]

The engine has 270,000 miles, and oil changes were consistently performed every 9,000 miles. It produces a rattling sound for 1-2 seconds on cold starts. The EGR and intake were excessively coated with residue, so I used Seafoam to clean them. Today, I am replacing the coils and spark plugs. Additionally, administering injector cleaner.

Cylinders 1 and 4 are excessively clean, suggesting potential coolant infiltration. The compression is 120 psi in all four cylinders. Perhaps they were cleaned using a gasoline additive?

If the head is damaged, I will replace the engine. I want a definitive diagnosis. Any feedback is appreciated, thank you.

[Talentrews]

A reliable diagnosis cannot be obtained by a forum text message.

This may also result from oil collection in the intake manifold being drawn into the cylinders during initiation. Additionally, valve stem seals.

What is the condition of the hybrid battery?
If you replace the engine, you may also anticipate a battery replacement costing around $2000 in the near future.
Prior to investing significant time and resources, I highly recommend obtaining a diagnosis from a reputable shop, maybe two.

[Jollyno]

The hybrid battery was changed after 190,000 miles. Charges efficiently as shown by the Dr. Prius application. Unconcerned over the matter.

No coolant is expelled via the spark plug wells when they are devoid of fluid. The coils and plugs were aged, so I replaced them.

The EGR is completely clean and functioning properly. I will wait to confirm it is resolved after 10 cold starts without any rattling.

Thank you.

[Talentrews]

The PCV system, which might be a source of oil if pooling occurs in the intake manifold, was not mentioned.

[Jollyno]

Thank you for the reminder; I have bought a PCV valve and will replace it as soon as possible. I have delayed the removal of the intake due to the difficulty encountered with the EGR and cooler removal.

I introduced Seafoam into the EGR pipe, allowing it to traverse the intake ports. Expelled a significant amount of debris from the spark plug wells and exhaust system.

[Talentrews]

Best of luck. Engaging in such actions might result in more harm than benefit.

[Jollyno]

It is a plastic intake; nonetheless, there are few aspects that may be further deteriorated. The EGR pipe connects to the tiny ports on the intake and subsequently to the manifold. Seafoam is formulated particularly for injector cleaning, making it entirely safe for use in cylinders. The obstructed channels undoubtedly expelled debris and carbon along with the Seafoam. I removed the spark plugs and cranked the engine, which expelled the debris from the top of the engine instead of obstructing the catalytic converter. Upon reassembling the EGR and reinserting the plugs and coils, I emitted a substantial cloud of white smoke.

The vehicle is operating much better; it has not shown rattling throughout six cold starts, the persistent OBD misfire errors have been eliminated, and power has been enhanced.

[Talentrews]

It is not.
Seafoam was developed as a fuel stabiliser.
If executed improperly, such a process may amass sufficient liquid in a cylinder to harm the engine.

[Jollyno]

The liquid inside the cylinder must be pressurised to inflict injury. I removed the plugs and coils. The Seafoam traversed the intake, passed through the valves, and exited the top of the cylinder. It is difficult to conceive of any harm resulting from it. The little amount of Seafoam that entered the exhaust likely facilitated carbonisation as well.

[Jollyno]

[Talentrews]

Negative. It has already sent sufficient marketing exaggerations and falsehoods.
I resign.